Multi-rise time PWM: A way to reduce PD exposure in motor windings

Journal article, 2020

Decreasing the rise time of pulse width modulated waveforms (PWM) has been observed to reduce the life length of high voltage motors. To solve this problem, the advantages of employing multilevel PWM waveforms with adjustable rise times operating above partial discharge inception are discussed in this paper. Examples of how the voltage supply circuits can be designed to enable investigation of this well as measured results are presented. To elucidate this approach, it is investigated what influence the choice of inverter levels has on the partial discharge (PD) characteristics within motor winding material applying adjustable rise times. The test objects were fed from either three- or four level inverter voltage waveforms to resemble realistic stress conditions as in field application. Presented experimental results show that the total peak and summed PD magnitude (exposure) drops considerably when applying a longer rise time at the critical voltage flanks and is shown to reduce the wear of the winding insulation material, showing the consistency between PD pattern and wear. It is further suggested that the presence of PDs can be eliminated by adjusting the rise times at the most critical voltage flanks and thus enhance the life of the machine winding. The presented principles can be implemented for various PWM pattern.